Aberrant DNA methylation is an epigenetic mechanism that can inactivate the expression of genes that suppress tumorigenesis. The genes involved include tumor suppressor genes; genes that suppress apoptosis, metastasis and angiogenesis; genes that repair DNA; and genes that express tumor-associated antigens. The molecular mechanism of silencing gene expression appears to be due to the attachment of 5-methylcytosine binding proteins to the methylated promoter, which blocks the action of transcription factors (see FIG. 1).
Because this epigenetic change is reversible, it presents an interesting target for chemotherapeutic intervention. 5-Azacytidine was the first hypomethylating agent approved by the FDA for the treatment of a neoplasm and the deoxy-analog, 5-azadeoxycytidine or decitabine, was more recently approved for the same indication (see FIG. 2). Both drugs produce remissions or clinical improvements in more than half of the treated patients with myelodysplatic syndrome (MDS). Features of responses include the requirement for multiple cycles of therapy, slow responses, and actual clonal elimination. Optimization of therapy has included (1) reducing the dose to favor hypomethylation over cytotoxicity, (2) prolonging administration schedules, and (3) increasing dose intensity without reaching cytotoxicity. Molecularly, hypomethylation and gene reactivation have been shown and seem to be required for responses. The data in MDS represent a proof-of-principle for epigenetic therapy. Although the therapy is effective, with complete responses lasting months to years in some patients, resistance seems to develop in the majority of patients, and the mechanisms of resistance are unknown.
Data from the two currently approved drugs suggest that myeloid malignancies are the neoplasms most sensitive to inhibitors of DNA methylation. However, there is no known reason why solid tumors should not respond as well. Recently, preclinical studies with DNA methyl transferase inhibitors (DNMTIs) have demonstrated that they are also potent angiostatic agents, inhibiting tumor endothelial cells and angiogenesis in vitro and in vivo, adding to the rationale for the treatment of solid tumors with these agents.
5-Azacytidine and decitabine are known to be unstable in water, with cleavage of the base at the 6-position of the cytosine ring (see FIG. 3, IIIa). Both nucleosides are known to suffer from a short half-life. The half-life of 5-azacytidine following subcutaneous administration is reported to be 41 minutes and that of decitabine 30 minutes. Both drugs are good substrates for cytidine deaminase and the short half-life is primarily due to deamination of the base to inactive uracil.
Gemcitabine differs from cytidine by a difluorinated sugar at the 2′-position. See FIG. 4.
The cytotoxic effect of gemcitabine is attributed to a combination of two actions of the diphosphate and the triphosphate nucleosides, which leads to inhibition of DNA synthesis. First, gemcitabine diphosphate inhibits ribonucleotide reductase, which is responsible for catalyzing the reactions that generate the deoxynucleoside triphosphates for DNA synthesis. Inhibition of this enzyme by the diphosphate nucleoside causes a reduction in the concentrations of deoxynucleotides, including dCTP. Second, gemcitabine triphosphate competes with dCTP for incorporation into DNA. The reduction in the intracellular concentration of dCTP (by the action of the diphosphate) enhances the incorporation of gemcitabine triphosphate into DNA (self-potentiation). After the gemcitabine nucleotide is incorporated into DNA, only one additional nucleotide is added to the growing DNA strands. After this addition, there is inhibition of further DNA synthesis. DNA polymerase epsilon is unable to remove the gemcitabine nucleotide and repair the growing DNA strands (masked chain termination).
There exists a need for novel cancer therapeutics with improved efficacy, safety, and/or pharmacokinetic profiles. The invention provides novel fluorinated pyrimidine analogs and compositions for the treatment of cancer.